Gallium(III)- and Indium(III)-Containing Ionic Liquids as Highly Active Catalysts in Organic Synthesis

The chemical industry still requires development of environmentally friendly processes. Acid-catalysed chemical processes may cause environmental problems. Urgent need to replace conventional acids has forced the search for sustainable alternatives. Metal-containing ionic liquids have drawn considerable attention from scientists for many years. These compounds may exhibit very high Lewis acidity, which is usually dependent on the composition of the ionic liquid with the particular content of metal salt. Therefore, metal-containing ionic liquids have found a lot of applications and are successfully employed as catalysts, co-catalysts or reaction media in various fields of chemistry, especially in organic chemistry. Gallium(III)- and indium(III)-containing ionic liquids help to transfer the remarkable activity of metal salts into even more active and easier-to-handle forms of ionic liquids. This review highlights the wide range of possible applications and the high potential of metal-containing ionic liquids with special focus on Ga(III) and In(III), which may help to outline the framework for further development of the presented research topic and synthesis of new representatives of this group of compounds.


Introduction
Ionic liquids (ILs) are organic salts that consist entirely of ions, formed by combining organic cations and inorganic or organic anions, resulting in compounds presenting melting points below 100 • C. The most significant ILs properties, such as negligible vapour pressures and very wide liquidus range, often greater than 200 • C, are of high interest to organic chemists. Furthermore, ILs have the ability to dissolve many organic and inorganic compounds and hence are frequently used as solvents [1][2][3][4][5]. The main problem of many crucial chemical processes arises from the use of hazardous acids as catalysts. Conventional approaches are associated with some issues, e.g., generation of a large amount of waste. Therefore, a growing interest in research concerning alternative catalysts, which should be active, selective and stable as well as solve problems resulting from the use of conventional acids, is of high importance [6][7][8]. On the other hand, salts of various metals, such as boron, aluminium, gallium, indium or noble metals, e.g., copper, are also successfully used in catalysis [9][10][11][12][13][14], especially in the form of halometallates, because they represent high Lewis acidity, which distinguishes them from other groups of salts. The subjects of this review are metal-based ionic liquid focusing on gallium(III) and indium(III) and their performance in organic chemistry.

Acidity by Gutmann Acceptor Number
The Gutmann Acceptor Number (AN) is a well-known quantitative technique for measurement of acidity of compounds that are off the pH scale or inadequate for Hammett acidity function. The AN method is based on 31 P NMR chemical shift measurements for samples containing triethylphosphine oxide (TEPO) dissolved in the tested substance. The complexation of TEPO with acid influences the chemical shift in the 31 P NMR spectrum, based on which the acceptor number is determined. Using this method, the AN of hexane was fixed to be zero, and the acidities for other solvents and classical acids were determined from this value, as shown in Table 1 [76]. Table 1. Acceptor numbers of common substances [59]. AN measurements are regularly performed for Lewis acid-based ILs to determine their acidic properties. For chlorometallate ILs, the commonly used citation is 1-octyl-3-methylimidazolium chloride ([C 8 mim]Cl) with adequate metal chlorides in various compositions χMCl 3 (χ = molar fraction) from 0.25 to 0.75 [29,30]. The obtained results are presented in Table 2. The ILs based on chlorogallate(III) show the highest AN among the tested ILs. ILs with χGaCl 3 = 0.75 exhibit acidity comparable to AN of trifluoroethanoic acid. On the other hand, chloroindate(III) systems were reported to be mild Lewis acids or even characterized as neutral. After all, one of the most interesting aspects of the acidity of the measured chlorometallate(III) ILs is their function as Lewis acids in neutral compositions (χMeCl 3 = 0.50), which is described below. Considering their better hydrolytic stability than aluminium-based systems, chlorogallate(III) and chloroindate(III) ILs may have a strong advantage in catalysis, where highly acidic or medium acidic conditions are required [38,39]. Table 2. Series of anionic spicies and acceptor numbers for selected chlorometallate(III) ionic liquids based on [C 8 mim] + cation [38,39,41]. The ILs based on metal chlorides are a well-known group characterised with strong Lewis acidity. The strength of the Lewis acid depends on the radius of the metalloid; thus, for a series of chlorometallates (MCl 3 ) containing metals from Group IIIA, their strength will increase in the following order: In < Ga < Al. The influence of chosen metal on acidity of ILs was frequently confirmed, e.g., Angueira [77].

Ga(III)-Based Ionic Liquids
Developing the topic of halometallate ILs, Yong et al. reported the application of [C 4 mim][GaCl 4 ] as an active and efficient catalyst for the acetalization of aldehydes (Figure 1). In the case of conducting the reaction between methanol and benzaldehyde, it was possible to obtain the product with 81% yield in the presence of 5 mol% catalyst at room temperature. In comparison, the use of neat salt enables the yield of up to 55% after the same time and reaction conditions. Following the procedure, the authors also carried out the acetalization of acetaldehyde and propionaldehyde, coming up with corresponding acetals with 97% and 98% yields, respectively. In addition, the study proved that the catalyst could be used for at least five cycles without notable loss of efficiency [78].

Ga(III)-Based Ionic Liquids
Developing the topic of halometallate ILs, Yong et al. reported the application of [C4mim][GaCl4] as an active and efficient catalyst for the acetalization of aldehydes (Figure 1). In the case of conducting the reaction between methanol and benzaldehyde, it was possible to obtain the product with 81% yield in the presence of 5 mol% catalyst at room temperature. In comparison, the use of neat salt enables the yield of up to 55% after the same time and reaction conditions. Following the procedure, the authors also carried out the acetalization of acetaldehyde and propionaldehyde, coming up with corresponding acetals with 97% and 98% yields, respectively. In addition, the study proved that the catalyst could be used for at least five cycles without notable loss of efficiency [78].  Figure 2). Tetrachlorogallate IL was used as a reaction medium and PdCl2(PPh3)2 as a catalyst. The ethoxycarbonylation of styrene led to two regioisomers, linear and branched esters. During the reaction in the presence of PdCl2(PPh3)2, the regioselectivity to branched ester achieved 77% and 67% yield, while the linear ester was obtained with 20% yield. After the addition of 1,1'-bis(diphenylphosphino)ferrocene (dppf) to a palladium catalyst, the regioselectivity of the process drastically changed. The branched ester was formed with 38% selectivity and 26% yield. On the other hand, in this catalytic system, the linear product yielded 43% [79]. Reports describing ILs containing a gallium atom and relatively large organic cations, such as 1-butyl-isoquinolinium gallium tetrachloride [(BIQL)GaCl4], are also known but without exemplary application [80].
Atkins et al. demonstrated the application of 1-ethyl-3-methylimidazolium heptachlorodigallate(III) [C2mim][Ga2Cl7] in the oligomerization process of linear olefins. Performed studies concerned 1-pentene transformation into a C20-C50 blend, which was used as a lubricant-based oil. Preliminary experiments revealed C20-C50 products with 6% conversion and 58% selectivity. Based on the initial data, the authors simulated the optimization of the conditions, according to which the selection of appropriate process parameters helped to achieve conversion of up to 61% and selectivity to the desired products at the level of 40% [81].
Alkylation is an important refinery process in the petroleum industry, which helps to produce large quantities of highly branched chain paraffin hydrocarbons. One of the  Figure 2). Tetrachlorogallate IL was used as a reaction medium and PdCl 2 (PPh 3 ) 2 as a catalyst. The ethoxycarbonylation of styrene led to two regioisomers, linear and branched esters. During the reaction in the presence of PdCl 2 (PPh 3 ) 2 , the regioselectivity to branched ester achieved 77% and 67% yield, while the linear ester was obtained with 20% yield. After the addition of 1,1'-bis(diphenylphosphino)ferrocene (dppf) to a palladium catalyst, the regioselectivity of the process drastically changed. The branched ester was formed with 38% selectivity and 26% yield. On the other hand, in this catalytic system, the linear product yielded 43% [79].

Ga(III)-Based Ionic Liquids
Developing the topic of halometallate ILs, Yong et al. reported the application of [C4mim][GaCl4] as an active and efficient catalyst for the acetalization of aldehydes (Figure 1). In the case of conducting the reaction between methanol and benzaldehyde, it was possible to obtain the product with 81% yield in the presence of 5 mol% catalyst at room temperature. In comparison, the use of neat salt enables the yield of up to 55% after the same time and reaction conditions. Following the procedure, the authors also carried out the acetalization of acetaldehyde and propionaldehyde, coming up with corresponding acetals with 97% and 98% yields, respectively. In addition, the study proved that the catalyst could be used for at least five cycles without notable loss of efficiency [78].  Figure 2). Tetrachlorogallate IL was used as a reaction medium and PdCl2(PPh3)2 as a catalyst. The ethoxycarbonylation of styrene led to two regioisomers, linear and branched esters. During the reaction in the presence of PdCl2(PPh3)2, the regioselectivity to branched ester achieved 77% and 67% yield, while the linear ester was obtained with 20% yield. After the addition of 1,1'-bis(diphenylphosphino)ferrocene (dppf) to a palladium catalyst, the regioselectivity of the process drastically changed. The branched ester was formed with 38% selectivity and 26% yield. On the other hand, in this catalytic system, the linear product yielded 43% [79]. Reports describing ILs containing a gallium atom and relatively large organic cations, such as 1-butyl-isoquinolinium gallium tetrachloride [(BIQL)GaCl4], are also known but without exemplary application [80].
Atkins et al. demonstrated the application of 1-ethyl-3-methylimidazolium heptachlorodigallate(III) [C2mim][Ga2Cl7] in the oligomerization process of linear olefins. Performed studies concerned 1-pentene transformation into a C20-C50 blend, which was used as a lubricant-based oil. Preliminary experiments revealed C20-C50 products with 6% conversion and 58% selectivity. Based on the initial data, the authors simulated the optimization of the conditions, according to which the selection of appropriate process parameters helped to achieve conversion of up to 61% and selectivity to the desired products at the level of 40% [81].
Alkylation is an important refinery process in the petroleum industry, which helps to produce large quantities of highly branched chain paraffin hydrocarbons. One of the Reports describing ILs containing a gallium atom and relatively large organic cations, such as 1-butyl-isoquinolinium gallium tetrachloride [(BIQL)GaCl 4 ], are also known but without exemplary application [80].
Atkins et al. demonstrated the application of 1-ethyl-3-methylimidazolium heptachlorodigallate(III) [C 2 mim][Ga 2 Cl 7 ] in the oligomerization process of linear olefins. Performed studies concerned 1-pentene transformation into a C20-C50 blend, which was used as a lubricant-based oil. Preliminary experiments revealed C20-C50 products with 6% conversion and 58% selectivity. Based on the initial data, the authors simulated the optimization of the conditions, according to which the selection of appropriate process parameters helped to achieve conversion of up to 61% and selectivity to the desired products at the level of 40% [81].
Alkylation is an important refinery process in the petroleum industry, which helps to produce large quantities of highly branched chain paraffin hydrocarbons. One of the most essential variants is the alkylation of light oil olefin (C3-C5) and isobutane, to obtain mainly isopentan and isooctan, which are mostly used as a blending stock for reformulated gasoline. Xing et al. applied a series of triethylammonium-based chlorogallate(III) ILs ([Et 3 NHCl]-GaCl 3 ) with a molar fraction ratio of χGaCl 3 from 0.60 to 0.75. The system containing χGaCl 3 = 0.65 evaluated the highest product values among all investigated catalysts; therefore, it was next used with the addition of copper halide as a catalytic performance-improving additive [82,83]. As a result, it was possible to achieve better selectivity to C8 products up to 70.1 wt%, trimethylpentane up to 50.5 wt% and 91.3 wt% of total alkylate RON. Moreover, the research revealed the reusability of the catalytic system in at least nine cycles without any relevant loss in activity.
Among the possible applications, chlorogallate(III) Ils were also investigated as catalysts and media in alkylation of methyl linoleate with propene to prepare valuable branched high-molecular-weight products. Ga-based Ils were chosen due to their Lewis acid-catalytic ability and higher resistance to the presence of moisture than chloroaluminate Ils. was much more effective for propene oligomerization [84].
Oligomerization of 1-decene in the presence of a series of gallium(III) chloride and urea compositions was investigated to produce poly(α-olefins). Based on the data provided, an increase in the selectivity of the product was observed, due to the growing molar fraction of GaCl 3 from 0.50 to 0.75 in the final composition of the catalyst [55].
Another application of gallium-containing Ils in the alkylation reaction can be found for [C 2 mim][Ga 2 Cl 7 ]. In this Friedel-Crafts process, benzene and 1-decane were used as substrates for the model reaction. Among many catalytic options, the use of [C 2 mim][Ga 2 Cl 7 ] helped to obtain phenyldecanes with a very high 91% yield and selectivity at room temperature with catalyst loading at 1 mol%. In traditional approaches, the described process requires higher temperatures, up to 70-150 • C [85].
In another approach, [C 2 mim]Cl-GaCl 3 (χGaCl 3 = 0.50-0.75) was employed as a catalyst for Baeyer-Villiger oxidation. The use of hydrogen peroxide 30% aq. containing χGaCl3 = 0.65 evaluated the highest product values among all investigated catalysts; therefore, it was next used with the addition of copper halide as a catalytic performance-improving additive [82,83]. As a result, it was possible to achieve better selectivity to C8 products up to 70.1 wt%, trimethylpentane up to 50.5 wt% and 91.3 wt% of total alkylate RON. Moreover, the research revealed the reusability of the catalytic system in at least nine cycles without any relevant loss in activity.
Among the possible applications, chlorogallate(III) Ils were also investigated as catalysts and media in alkylation of methyl linoleate with propene to prepare valuable branched high-molecular-weight products. Ga-based Ils were chosen due to their Lewis acid-catalytic ability and higher resistance to the presence of moisture than chloroaluminate Ils. The implementation of [C4mim][GaCl4/Ga2Cl7] and [BuIsoq][GaCl4/Ga2Cl7] shows different catalytic performances. In case of the use of [C4mim][GaCl4/Ga2Cl7], apart from the alkylation process, polypropene formation and oligomerization of methyl linoleate providing branched long-chain diesters and triesters was observed, whereas [Bu-Isoq][GaCl4/Ga2Cl7] was much more effective for propene oligomerization [84].
Oligomerization of 1-decene in the presence of a series of gallium(III) chloride and urea compositions was investigated to produce poly(α-olefins). Based on the data provided, an increase in the selectivity of the product was observed, due to the growing molar fraction of GaCl3 from 0.50 to 0.75 in the final composition of the catalyst [55].
Another application of gallium-containing Ils in the alkylation reaction can be found for [C2mim][Ga2Cl7]. In this Friedel-Crafts process, benzene and 1-decane were used as substrates for the model reaction. Among many catalytic options, the use of [C2mim][Ga2Cl7] helped to obtain phenyldecanes with a very high 91% yield and selectivity at room temperature with catalyst loading at 1 mol%. In traditional approaches, the described process requires higher temperatures, up to 70-150 °C [85].
In another approach, [C2mim]Cl-GaCl3 (χGaCl3 = 0.50-0.75) was employed as a catalyst for Baeyer-Villiger oxidation. The use of hydrogen peroxide 30% aq. for 2-adamantanone oxidation resulted in a very fast transformation. Reported attempts reveal the influence of GaCl3 content on the catalyst activity.   The studies were extended to explore ILs containing a borenium cation combined with 4-picoline (4pic), 1-methylimidazol (mim) or dimethylacetamide (dma) and [GaCl 4 ] − or [Ga 2 Cl 7 ] − as the counterion. For each of them, the maximum conversion of ethyl acrylate was achieved, despite that the less acidic ILs had to be used in larger amounts. However, full conversion was achieved in only 5 min and corresponded with an unchanged endo:exo (94:6) ratio by the diverse catalyst loading [67].
In The studies were extended to explore ILs containing a borenium cation combined with 4-picoline (4pic), 1-methylimidazol (mim) or dimethylacetamide (dma) and [GaCl4] − or [Ga2Cl7] − as the counterion. For each of them, the maximum conversion of ethyl acrylate was achieved, despite that the less acidic ILs had to be used in larger amounts. However, full conversion was achieved in only 5 min and corresponded with an unchanged endo:exo (94:6) ratio by the diverse catalyst loading [67].
In   Table 3 summarizes the applications of gallium(III)-based ILs as catalysts in organic synthesis.  Table 3 summarizes the applications of gallium(III)-based ILs as catalysts in organic synthesis.

In(III)-Based Ionic Liquids
The speciation of chloroindate(III) ionic liquids has been already described, including confirmation of the presence of [InCl 6

] 3− , [InCl 5 ] 2− and [InCl 4 ] − ions in compositions of ILs based on [C 4 mim]Cl and [C 8 mim]Cl in combination with InCl
One of the first works introducing indium-based ILs reveals their application as bifunctional catalysts performing as reaction media for Friedel-Crafts acylation reactions. The performance of the catalyst in benzoylation of anisole with benzoic anhydride was examined for [C 4 mim][InCl4] (χInCl 3 = 0.67) and was compared with the use of InCl 3 without a solvent, in 1,2-dichloroethane or with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C 4 mim][NTf 2 ]). The results turned out very valuable due to the very fast reaction progress for [C 4 mim][InCl 4 ] used as a catalyst and reaction medium. Moreover, the benzoylation of benzene and it derivatives such as methoxybenzene, toluene, isobutylbenzene was performed with benzoic anhydride, benzoyl chloride or ethanoic anhydride ( Figure 5) with 1.1-1.5 molar excess of the acylating agent. After 48 h of reaction, the yield for specific substrates oscillated around 81-97% with high 81-98% selectivity to the main products [90].

In(III)-Based Ionic Liquids
The speciation of chloroindate(III) ionic liquids has been already described, including confirmation of the presence of [InCl6] 3 [71].
One of the first works introducing indium-based ILs reveals their application as bifunctional catalysts performing as reaction media for Friedel-Crafts acylation reactions. The performance of the catalyst in benzoylation of anisole with benzoic anhydride was examined for [C4mim][InCl4] (χInCl3 = 0.67) and was compared with the use of InCl3 without a solvent, in 1,2-dichloroethane or with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim][NTf2]). The results turned out very valuable due to the very fast reaction progress for [C4mim][InCl4] used as a catalyst and reaction medium. Moreover, the benzoylation of benzene and it derivatives such as methoxybenzene, toluene, isobutylbenzene was performed with benzoic anhydride, benzoyl chloride or ethanoic anhydride ( Figure 5) with 1.1-1.5 molar excess of the acylating agent. After 48 h of reaction, the yield for specific substrates oscillated around 81-97% with high 81-98% selectivity to the main products [90]. In another approach, indium(III)-containing ILs were studied in alcohol protection by transferring them to tetrahydropyranyl ethers. Herein, 1-alkyl-3-methylimidazolium tetrachloroindate(III) ILs were very efficient and recyclable catalysts. The tetrahydropyranylations reaction of various alcohols was performed using a microwave (MW) power unit for 5 min instead of conventional heating at 60 °C per 1 h. Carrying out the reaction in the presence of 25 mol% of [C4mim][InCl4] enabled improved output of the obtained ethers from phenol and benzyl alcohols from 20% and 39% yields to 88% and 85%, respectively. It was found that the impact of changing of the alkyl group attached to the imidazolium cation in IL was negligible in terms of catalytic activity in the model benzyl alcohol transition to ether. Additional studies proved not only high effectiveness of [C4mim][InCl4] in tetrahydropyranylation of cinnamyl alcohol, 1-phenylethanol, 2-phenylethanol, 3-methylpent-4-enol, but also impressive stability within five cycles without the loss of benzyl alcohol yield [91].
The R.S. Varma team further explored the perspective application of chloroindate(III) ILs and in the same year published the results of using [C4mim][InCl4] as a catalyst for the acetalization reaction of aromatic aldehydes with methanol. Replacing indium(III) chloride with 5 mol% of IL and carrying out the reaction at room temperature for 30 min helped to increase the yield of product from 40% to 70% [78].
The work by Varma et al. describes the series of haloindate(III)-based ionic liquids and their use as highly active catalysts for epoxides coupled with CO2 in order to produce cyclic carbonates ( Figure 6). The catalytic efficiency was evaluated in propylene oxide coupling with CO2, conducting the process for 1 h in the presence of 5 mol% of the catalyst. (all with χInX3 = 0.5) helps to achieve up to 94% yield with full selectivity, whereas the usage of neat InCl3 did not initiate this reaction. Furthermore, changing the cation structure into 3-butyl-1,2-dimethylimidazolium (bdmim) resulted in increasing the yield to In another approach, indium(III)-containing ILs were studied in alcohol protection by transferring them to tetrahydropyranyl ethers. Herein, 1-alkyl-3-methylimidazolium tetrachloroindate(III) ILs were very efficient and recyclable catalysts. The tetrahydropyranylations reaction of various alcohols was performed using a microwave (MW) power unit for 5 min instead of conventional heating at 60 • C per 1 h. Carrying out the reaction in the presence of 25 mol% of [C 4 mim][InCl 4 ] enabled improved output of the obtained ethers from phenol and benzyl alcohols from 20% and 39% yields to 88% and 85%, respectively. It was found that the impact of changing of the alkyl group attached to the imidazolium cation in IL was negligible in terms of catalytic activity in the model benzyl alcohol transition to ether. Additional studies proved not only high effectiveness of [C 4 mim][InCl 4 ] in tetrahydropyranylation of cinnamyl alcohol, 1-phenylethanol, 2-phenylethanol, 3-methylpent-4-enol, but also impressive stability within five cycles without the loss of benzyl alcohol yield [91].
The R.S. Varma team further explored the perspective application of chloroindate(III) ILs and in the same year published the results of using [C 4 mim][InCl 4 ] as a catalyst for the acetalization reaction of aromatic aldehydes with methanol. Replacing indium(III) chloride with 5 mol% of IL and carrying out the reaction at room temperature for 30 min helped to increase the yield of product from 40% to 70% [78].
The work by Varma et al. describes the series of haloindate(III)-based ionic liquids and their use as highly active catalysts for epoxides coupled with CO 2 in order to produce cyclic carbonates ( Figure 6). The catalytic efficiency was evaluated in propylene oxide coupling with CO 2 , conducting the process for 1 h in the presence of 5 mol% of the catalyst. The application of 1-butyl-3-methylimidazolium haloindate(III)-based ILs, such as [C 4 3 Cl] (all with χInX 3 = 0.5) helps to achieve up to 94% yield with full selectivity, whereas the usage of neat InCl 3 did not initiate this reaction. Furthermore, changing the cation structure into 3-butyl-1,2-dimethylimidazolium (bdmim) resulted in increasing the yield to 95% and replacing it with the n-butylpyridinium (bPy) cation, enabling the highest yield of 97% [89].
Gunaratne et al. proved that chloroindate(III) ILs are versatile and efficient catal for alkylation of phenol and their derivatives with alkenes. The studies employed vari alkenes, such as propene, isobutene, diisobutene and 2-methylheptene. Interposed catalyst, In(III)-based IL was in the form of [C4mim][InCl4] (χInCl3 = 0.67). The reactio phenol with diisobutene and 2-methylheptene led to the main products, which wer substituted phenol derivatives with high 83% and 81% yields, respectively. Howe with more reactive alkene, such as isobutene, the reaction was driven to the main prod as disubstituted phenol (2,4-di-tert-butylphenol) with 78% yield, whereas the replacem of phenol with p-cresol in the reaction with isobutene led to as 78% yield to monosub tuted 2-tert-butyl-4-methylphenol. Furthermore, it turned out that all tested reacti with propene as the substrate were characterised with a non-selective distribution products. The study was extended to explore the catechol alkylation with isobutene diisobutene. The results showed equally high yields up to 85% and 88%, accordingly [ Another research focused on the synthesis of 3,4-dihydropyrimidinones in colla ration with [C4mim][InCl4] as an efficient catalyst. Conducted studies provided a serie 18 different Biginelli condensation products with very high yields (82-98%), proving wide effectiveness of the IL [94]. The same catalytic system was examined for the addition of hydrazones to diketones (Figure 7). [C4mim][InCl4] was applied in regioselective, one-pot condensa of aldehydes, arylhydrazines and acyclic or cyclic 1,3-diketones to synthesised pyrazo Performed attempts revealed an overwhelming advantage of In(III)-based IL, especi in the field of the regioselectivity of reaction between benzaldehyde, phenylhydrazine 1-phenylbutane-1,3-dione, due to the formation of just one regioisomer. The hig achieved isolated yield was 86% with the presence of only one regio-product. Expand the scope of the tested substrates to substituted arylhydrazine and various aldehydes, only confirmed the effectiveness of the proposed catalytic system but also the universa of the method [95]. Another report concerned the application of the same catalytic system established on [C 4 mim][InCl 4 ] (χInCl 3 = 0.50) in the addition of allyltrimethylsilane, silyl enol ethers and ketene silyl acetals to cyclic N-acyliminium ions gain from carbamate N-Boc-2-methoxypyrrolidine (αmethoxycarbamate). Reactions carried out in chloroindium(III)-based IL helped to obtained product addition of allyltrimethylsilane and N-Boc-2-methoxypyrrolidinehigh with 80% yield, 68% yield for the addition of allyltrimethylsilane and piperidine derivative and a very high 89% yield for the allyltrimethylsilane and α-methoxytetrahydroisoquinoline reaction, whereas from silyl enol ethers and α-methoxycarbamate, it was possible to obtain products with 65-78% yields, and for ketene silyl acetal and various α-methoxycarbamate, the yields were in the range from 67% to 79% [92].
Gunaratne et al. proved that chloroindate(III) ILs are versatile and efficient catalysts for alkylation of phenol and their derivatives with alkenes. The studies employed various alkenes, such as propene, isobutene, diisobutene and 2-methylheptene. Interposed as a catalyst, In(III)-based IL was in the form of [C 4 mim][InCl 4 ] (χInCl 3 = 0.67). The reaction of phenol with diisobutene and 2-methylheptene led to the main products, which were 4-substituted phenol derivatives with high 83% and 81% yields, respectively. However, with more reactive alkene, such as isobutene, the reaction was driven to the main product as disubstituted phenol (2,4-di-tert-butylphenol) with 78% yield, whereas the replacement of phenol with p-cresol in the reaction with isobutene led to as 78% yield to monosubstituted 2-tert-butyl-4-methylphenol. Furthermore, it turned out that all tested reactions with propene as the substrate were characterised with a non-selective distribution of products. The study was extended to explore the catechol alkylation with isobutene and diisobutene. The results showed equally high yields up to 85% and 88%, accordingly [93].
Another research focused on the synthesis of 3,4-dihydropyrimidinones in collaboration with [C 4 mim][InCl 4 ] as an efficient catalyst. Conducted studies provided a series of 18 different Biginelli condensation products with very high yields (82-98%), proving the wide effectiveness of the IL [94]. The same catalytic system was examined for the addition of hydrazones to 1,3diketones ( Figure 7). [C 4 mim][InCl 4 ] was applied in regioselective, one-pot condensation of aldehydes, arylhydrazines and acyclic or cyclic 1,3-diketones to synthesised pyrazoles. Performed attempts revealed an overwhelming advantage of In(III)-based IL, especially in the field of the regioselectivity of reaction between benzaldehyde, phenylhydrazine and 1-phenylbutane-1,3-dione, due to the formation of just one regioisomer. The highest achieved isolated yield was 86% with the presence of only one regio-product. Expanding the scope of the tested substrates to substituted arylhydrazine and various aldehydes, not only confirmed the effectiveness of the proposed catalytic system but also the universality of the method [95].  Several publications focused on biomass conversion into useful bio-based chemicals. Tiong et al. for the first time in 2017 presented the two-step method for conversions of oil palm empty fruit bunch (OPEFB) and mesocarp fibre (OPMF) biomass to levulinic acid and further esterification to ethyl levulinate [96]. The applied IL was described as eco-friendly, indium trichloride-1-methylimidazolium hydrogen sulphate, [Hmim][HSO 4 ]-InCl 3 . In 2019, the authors again approached the topic and updated the study within the optimisation of operating parameters and critical comparison of the potential of the proposed methodology with different catalysts reported in the literature. During the experiments, parameters such as biomass depolymerization temperature (177 • C) and esterification temperature (105 • C) were selected; the first stage was carried out for 4.8 h, while the second stage took 12.2 h. The maximum yield of levulinic acid reached 17.7% (from OPEFB) and 18.4% (from OPMF), along with an efficiency of 74.1% and 86.4%, respectively. Additionally, in the esterification of levulinic acid, the ethyl ester was obtained with 18.7% yield (from OPEFB) and 20.1% (from OPMF), corresponding to the efficiency of 63.2% and 75.3% for OPEFB and OPMF. Apart from that, the catalytic system exhibited the capability to be recycled. Up to three following runs maintaining 75% efficiencies for both biomass transformations were performed [97]. The described applications of indium(III)-based ILs as catalysts are shown in Table 4.

Other Applications of Ga(III) and In(III)-Based Ionic Liquids
Chlorogallate(III) ILs were also frequently used for gallium electrodeposition [98,99]. Additionally, haloindate(III) ionic liquids have been reported as a precursor to indium(0) nanoparticle production through an electrochemical reduction [100].
The latest publication from 2021 studied ILs based on the tetrakis(pentafluoroethyl)gallate anion, [Ga(C 2 F 5 ) 4 ] − [101]. These ILs with weakly coordinating anions were first presented for the first time in 2019, and the properties focusing on the potential electrochemical applications as conductive salts or additives to lithium-ion batteries were reported [102]. Gallate-based ILs contain various alkyl chain lengths at the imidazolium core, which is also combined with phenyl or phenyl derivatives and compared with [C 2 mim][Ga(C 2 F 5 ) 4 ] and [C 4 mim][Ga(C 2 F 5 ) 4 ]. New ILs featuring aryl-alkyl imidazolium cations combined with the tetrakis(pentafluoroethyl)gallate anion exhibited relatively low viscosities from 66 to 139 cP at 25 • C and exceptionally good electrochemical windows up to 5.15 V. On the other hand, the [C 2 mim] + and [C 4 mim] + based ILs showed very low viscosities of up to 29 cP at 25 • C but turned out to be electrochemically unstable [101].
A recently published paper provided information concerning gallium complexes created from methylidenemalonates and GaX 3 with a strict 3/4 composition, which are postulated to be ILs. The object of the research, apart from the demonstration of interactions occurring in the complex, was the three-component addition to a triple bond that led to the generation of polyfunctional vinyl halides through Ga(III)-mediated halides [103].
This year, interesting research has appeared demonstrating the effective application of chlorogallate(III) IL in the process of desulfurization. Trihexyl(tetradecyl)phosphonium tetrachlorogallate, [P 66614 ][GaCl 4 ] was used as a catalyst for H 2 S conversion into sulphur. The reaction carried out by oxidative absorption of H 2 S in IL for 6 h at 100 • C enabled the removal of 92% of hydrogen sulphide. Other ILs tested by the authors which contained Fe(III) or Sn(II) showed even better performance, reaching conversions up to 97%. Nevertheless, the presented studies showed that ILs based on metal chlorides can also be efficient tools in gas purification processes [104].
The immobilization facilitates the efficient recycling of catalysts and enhances the stability of ILs deposited onto solid carriers [105]. In addition to the described above applications, ILs containing metals of group 13 (Al, Ga, In) are also successfully used as active phases immobilized on solid supports, such as amorphous silica, MCM-41, mesoporous organosilica SBA-15 or carbon nanotubes [51,61]. They form supported ionic-liquid phases (SILPs) and silica-based supported ionic liquid-like phase (SILLP)-type systems, which have recently become very popular as heterogeneous catalysts in various chemical processes, such as alkylated gasoline production [106], Diels-Alder cycloaddition [51,87], Friedel-Crafts reactions [107] and alkylation [108][109][110], isobutene oligomerization [111] or even the synthesis of β-keto enol ethers [112] and their use is the subject of numerous research and review publications [107,113,114].

Conclusions
Currently, it is crucial to focus on the replacement of conventional catalysts due to the development of environmentally and economically sustainable processes. Based on the literature results, in most cases, the unique properties of ILs helped not only to improve the performance of catalysts, yield and selectivity or to mitigate the reaction conditions, but also enabled easier and more economical separation of products from the reaction mixtures.
According to the reviewed gallium-and indium-based ILs, described in the literature systems include mainly halometallic anions incorporated into the IL structure. Considering the wide spectrum of possible applications of ILs, mostly in organic catalysis and the extraordinary and ambiguous properties of group 13 metals, the creation of new systems that could be built from other anionic units containing these metals appears to be extremely interesting and perspective.